This invention relates to a method and an apparatus for manufacturing glass products by controlling free flow of flow viscosity molten glass so as to delay starting of free flow after cutting of the low viscosity molten glass flowing continuously from a feeder outlet.
There are many optical glasses which have tendency to devitrification during the melting process. For securing a required quantity of product while preventing occurrence of devitrification in forming such glasses into desired products, it has been customary to maintain the glass at an extremely low viscosity in the order of about 10 to 100 poises, cause this low viscosity glass to flow out of a feeder outlet of a relatively small diameter at a relatively high speed, cut the glass flow for obtaining a predetermined volume of gob and form this gob into the desired product. If a pair of shear blades which are normally used for cutting the flow of a relatively high viscosity glass are utilized for cutting the flow of the low viscosity glass, the glass tends to spatter or the glass flow tends to be disturbed with a result that defects such as folds or seams occur in the gob.
Various proposals have been made for cutting the glass flow without using the shear blades and thereby producing homogeneous gob. Japanese Patent Publication No. 24525/1976, for example, discloses a method for forming a gob by pouring glass flowing from the feeder outlet into a mould and moving the glass in the mould rapidly downwardly for cutting the glass flow when the glass has reached a predetermined amount. According to this method, however, a time interval between the cutting of the glass flow and restarting of flowing down of the glass from the feeder outlet is so short that it is not easy to set a mould for receiving a next gob of glass in position within this time interval. Besides, since the glass flow stretches long and assumes an unstable shape, occurrence of folds in produced gob cannot be sufficiently prevented.
Japanese Laid Open Patent Publication No. 50218/1978 discloses a method for forming a gob by cutting the glass flow by displacing a blade horizontally after pouring the glass flowing from a feeder outlet into a mould, holding the glass flow on the blade until a next mould is disposed beneath the feeder and then removing the blade from the glass flow to let the glass flow into the next mould. According to this method, intervention of the blade between the feeder and the mould creates an improper distance between them and gives rise to the above described disadvantages by a very rapid flowing down of the glass from the feeder immediately after cutting of the glass flow.